Also, if you are trying to compare two antennas of the same style (e.g. dipole), but different gain, then you will get something different. Let's keep the math easy and compare the pattern of a 100mw AP with a 3 dBi dipole antenna and a 50mw AP using a 6 dBi dipole antenna (assume no cable loss). The EIRP will be the same under both configurations (26 dBm / 400mw, if my math is right), but the coverage pattern will look very different.
If you think of a typical dipole as providing a coverage pattern that looks like a bagel (yes, I'm eating breakfast), then the bagel gets "squished" (spreads horizontally and reduced vertically) as you add antenna gain. Therefore, even though the EIRP would be the same in the example above, the configuration using he 6 dBi dipole will be wider/flatter than the configuration using the 3 dBi dipole.
Antenna gain doesn't add more power to the AP's signal - they are not powered amplifiers. I think the best way to describe gain is a "focusing" of the power.